Walls of the enterococci may represent 27 to 38% of the dry cell weight (exponential and stationary phase cells, respectively). Three main constituents are generally reported: peptidoglycan (PG), teichoic acid, and polysaccharide. Sometimes, proteins are also mentioned. Most of the ultrastructural analyses of the enterococcal cell walls were conducted in Enterococcus hirae ATCC9790. Structure, biosynthesis, and assembly of the different polymers that constitute the enterococcal cell walls are discussed in this chapter. The backbone of the enterococcal wall is PG, which is organized as a fisherman's net. One of the major functions of the PG in gram-positive organisms is the resistance to bursting induced by high cytoplasmic osmotic pressures. Many details of cell wall synthesis by enterococci are known, thanks in large measure to the extensive study of the mechanisms underlying vancomycin resistance. Cell wall-associated proteins have most extensively been studied in staphylococci and streptococci. Three categories of surface proteins are usually distinguished: those that have a LPXTG motif and anchor at their C-terminal ends, those that bind by way of charge and/or hydrophobic interactions, and those that bind by their N-terminal end. Few proteins have been described that bind through charge and/or hydrophobic interactions. Enterococcus faecalis and Enterococcus faecium cells can exchange genetic material (plasmids) by conjugation processes induced by small peptide pheromones. Analysis of the gene sequences of E. faecalis indicated that the pheromones produced by plasmid-free strains originate from the signal sequences of apparent lipoprotein precursors.

Model of the cell wall of a gram-positive organism. The multilayered peptidoglycan covers the cytoplasmic membrane bearing embedded proteins and lipoteichoic acids. To the peptidoglycan are bound or associated teichoic acids (rods), polysaccharides (hexagons), and proteins (small and large spheres). Modified from reference 35 with permission.

10.1128/9781555817923/fig5-1_thmb.gif

10.1128/9781555817923/fig5-1.gif

Figure 1

Model of the cell wall of a gram-positive organism. The multilayered peptidoglycan covers the cytoplasmic membrane bearing embedded proteins and lipoteichoic acids. To the peptidoglycan are bound or associated teichoic acids (rods), polysaccharides (hexagons), and proteins (small and large spheres). Modified from reference 35 with permission.

Cross-linked peptidoglycan of enterococci. The A3α type with an (Ala)2-3 cross-bridge is found in E. faecalis. The A4 α type with a d-isoAsn cross-bridge is found in E. faecium, E. hirae, and several other species.

10.1128/9781555817923/fig5-2_thmb.gif

10.1128/9781555817923/fig5-2.gif

Figure 2

Cross-linked peptidoglycan of enterococci. The A3α type with an (Ala)2-3 cross-bridge is found in E. faecalis. The A4 α type with a d-isoAsn cross-bridge is found in E. faecium, E. hirae, and several other species.

Model of the cell wall surface enlargement of E. hirae. The equatorial wall band marks the site of wall synthesis. It is first notched at the same time as the nascent septum starts to grow down. The septum elongates and is concomitantly split apart to make a new (clear) peripheral wall. Finally, the septum closes up the central gap and divides the original cell in two compartments. Reproduced from reference 4 with permission.

10.1128/9781555817923/fig5-4_thmb.gif

10.1128/9781555817923/fig5-4.gif

Figure 4

Model of the cell wall surface enlargement of E. hirae. The equatorial wall band marks the site of wall synthesis. It is first notched at the same time as the nascent septum starts to grow down. The septum elongates and is concomitantly split apart to make a new (clear) peripheral wall. Finally, the septum closes up the central gap and divides the original cell in two compartments. Reproduced from reference 4 with permission.

Proposed model of peptidoglycan assembly by a double channeling of cell wall precursors. Channel A is primarily involved with the synthesis of new cross wall. Channel A is involved in the conversion of the cross wall into two layers of thickening peripheral wall. Reproduced from reference 74 with permission.

10.1128/9781555817923/fig5-5_thmb.gif

10.1128/9781555817923/fig5-5.gif

Figure 5

Proposed model of peptidoglycan assembly by a double channeling of cell wall precursors. Channel A is primarily involved with the synthesis of new cross wall. Channel A is involved in the conversion of the cross wall into two layers of thickening peripheral wall. Reproduced from reference 74 with permission.

Proposed pathway for the synthesis of the serotype capsular polysaccharide in E. faecalis. In the first step of polysaccharide biosynthesis, the monosaccharides (represented by the small circles) must be activated by linkage to a nucleotide diphosphate (displayed as a circle with an asterisk). The nucleotide diphosphate provides the necessary energy to catalyze the polymerization of the monosaccharides into the oligosaccharide subunit by the glycosyl transferases shown in step 2. As a third step in the biosynthesis of the polysaccharide, the membrane-bound ABC transport proteins shuttle the oligosaccharide subunit across the cell membrane to the cell wall, where the polysaccharide is anchored to the cell. The transport of additional oligosaccharide subunits allows polymerization into a growing polysaccharide chain.

10.1128/9781555817923/fig5-7_thmb.gif

10.1128/9781555817923/fig5-7.gif

Figure 7

Proposed pathway for the synthesis of the serotype capsular polysaccharide in E. faecalis. In the first step of polysaccharide biosynthesis, the monosaccharides (represented by the small circles) must be activated by linkage to a nucleotide diphosphate (displayed as a circle with an asterisk). The nucleotide diphosphate provides the necessary energy to catalyze the polymerization of the monosaccharides into the oligosaccharide subunit by the glycosyl transferases shown in step 2. As a third step in the biosynthesis of the polysaccharide, the membrane-bound ABC transport proteins shuttle the oligosaccharide subunit across the cell membrane to the cell wall, where the polysaccharide is anchored to the cell. The transport of additional oligosaccharide subunits allows polymerization into a growing polysaccharide chain.

Model for the organization of cell wall polymers in the cell wall of E. faecalis. The lipid-anchored lipoteichoic acid, also known as the streptococcal group D antigen, is shown protruding into the cell wall peptidoglycan. Shown anchored to N-acetylmuramic acid (MNAc) residues of the peptidoglycan are the integral cell wall teichoic acids and the hypothesized enterococcal species antigen. Anchored to the N-acetylglucosamine (GNAc) residues in the peptidoglycan and protruding out from the peptidoglycan is the serotype-specific capsular polysaccharide.

10.1128/9781555817923/fig5-8_thmb.gif

10.1128/9781555817923/fig5-8.gif

Figure 8

Model for the organization of cell wall polymers in the cell wall of E. faecalis. The lipid-anchored lipoteichoic acid, also known as the streptococcal group D antigen, is shown protruding into the cell wall peptidoglycan. Shown anchored to N-acetylmuramic acid (MNAc) residues of the peptidoglycan are the integral cell wall teichoic acids and the hypothesized enterococcal species antigen. Anchored to the N-acetylglucosamine (GNAc) residues in the peptidoglycan and protruding out from the peptidoglycan is the serotype-specific capsular polysaccharide.

103.Maekawa, S.,, and S.Habadera. 1996. Comparative distribution of the serotypes of Enterococcus faecalis isolated from the urine of patients with urinary tract infections and the feces of healthy persons as determined by the slide agglutination reaction. Kansenshogaku Zasshi70:168–174.

104.Maekawa, S.,, M.Yoshioka,, and Y.Kumamoto. 1992. Proposal of a new scheme for the serological typing of Enterococcus faecalis strains. Microbiol. Immunol.36: 671–681.

108.Massidda, O.,, R.Kariyama,, L.Daneo-Moore,, and G. D.Shockman. 1996. Evidence that the PBP5-synthesis repressor (psr) of Enterococcus hirae is also involved in the regulation of cell wall composition and other cell wall-related properties. J. Bacteriol.178:5272–5278.

142.Sapunaric, E.,, C.Franssen,, R.Stefanic,, A.Amoroso,, O.Dardenne,, and J.Coyette. Synthesis of the low-affinity PBP5 and cellular autolysis are not controlled by the psr gene in Enterococcus hirae. Submitted.

143.Satta, G.,, R.Fontana,, and P.Canepari. 1994. The two-competing site (TCS) model for cell shape regulation in bacteria: the envelope as an integration point for the regulatory circuits essential physiological events. Adv. Microb. Physiol.36:181–245.

157.Staudenbauer, W. L.,, E.Willoughby,, and J. L.Strominger. 1972. Further studies of the D-aspartic acid-activating enzyme of Streptococcus faecalis and its attachment to the membrane. J. Biol. Chem.247:5289–5296.

178.Yamashita, Y.,, K.Tomihisa,, Y.Nakano,, Y.Shimazaki,, T.Oho,, and T.Koga. 1999. Recombination between gtfB and gtfC is required for survival of a dTDP-rhamnose synthesis-deficient mutant of Streptococcus mutans in the presence of sucrose. Infect. Immun.67:3693–3697.